Throughout all the books after the idea of apparition has been introduced, it is made quite clear that apparition on Hogwarts grounds is impossible. But in Chapter 21, The Unknowable Room, Dobby and Kreacher come to report to Harry, they apparate and disapparate. Up until this point it had never been mentioned how house elves transport like that, but the word "disapparate" is used in this chapter.

Uhm,about that Malfoy basement thing in HP7:If the house was magically protected against (dis)apparition,(except the house elves) how come that when Bellatrix(?) summoned Voldemort to the house Harry was so afraid that Voldemort would soon be within (dis)apparition range and would apparate in the room?Or was the house's protection just against GOOD witches and wizards?